This invention relates to a selection device, and more particularly to a selection device that selects, for example, the outputs of electric cells in a D/A converter, and to various apparatuses using the selection device.
With a conventional digital/analog converter, Din (an integer) unit current cells are selected to produce a current output corresponding to the input digital signal Din. Selecting unit current cells an output current of Io=Icell.times.Din is produced, whereby digital/analog conversion is effected.
In general, current cells have errors due to variations. If an error in each current cell is *i, the error contained in Io will be expressed by the following equation (1): ##EQU1##
Therefore, a differential linear error DNL has the value expressed by equation (2), so that the conversion accuracy is determined with process variations such as variations of manufacturing processes and manufacturing machines being reflected directly in the D/A conversion error: EQU DNL=.epsilon..sub.i ( 2)
To perform high-accuracy conversion, this scheme either uses an expensive high-accuracy process or requires adjustment such as trimming, resulting in higher cost.
A method that has improved this disadvantage is a dynamic element matching method 1!.
In this method, if the conversion time is Ts and the number of bits is n.sub.DA, the current cell used will be changed at intervals of Ts/2.sup.nDA so that all of the cells may be used equally in each conversion. By doing this, the output charge Qout is expressed by the following equation (3): ##EQU2## where Ii is the output current of the ith cell expressed as: ##EQU3## and Iav is a constant expressed as: ##EQU4##
Because of this, an error in each current cell has an effect only on the gain error. This makes it possible to achieve high conversion accuracy even when there are variations in the cells. Namely, an error in each cell is averaged with respect to time, thereby improving the accuracy.
With the dynamic element matching method, however, each cell must be selected in 1/2.sup.nDA of the conversion time, so that the elements are required to operate at high speeds.
As described above, the conventional scheme has the disadvantage that the performance deteriorates significantly due to variations in the elements, such as variations in the current cells. In the case of the dynamic element matching method, the current cells must be changed at a high speed, making it difficult to achieve a high-speed conversion operation.